Silicon nitride ceramic having high fatigue life and high toughness

ABSTRACT

A sintered silicon nitride ceramic comprising between about 0.6 mol % and about 3.2 mol % rare earth as rare earth oxide, and between about 85 w/o and about 95 w/o beta silicon nitride grains, wherein at least about 20% of the beta silicon nitride grains have a thickness of greater than about 1 micron.

STATEMENT OF GOVERNMENT SUPPORT

This invention was developed with support from the U.S. government underDOE Contract No. DE-AC05-840R21400 and DOD Contract No. DAAL-91-C-0039.

This is divisional of copending application Ser. No. 08/136,691 filed onOct. 14, 1993.

BACKGROUND OF THE INVENTION

Ceramic materials are currently viewed as potential substitutes forsteel and other metals due to their superior strength at hightemperatures, thermal shock resistance, toughness, hardness and chemicaloxidation resistance. One particular ceramic, silicon nitride, has beenidentified for use in high temperature applications such as gas turbinesand diesel engines.

Two critical parameters of any silicon nitride ceramic are its hightemperature strength and its toughness. Typically, high temperaturestrength is inversely related to the amount of sintering aid used, whiletoughness is directly related to the presence of grains having a highaspect ratio, i.e., beta phase grains whose lengths are at least fourtimes their thickness.

Processing raw materials into a useful ceramic body commonly requires anumber of engineering operations. A typical process entails preparing asilicon nitride powder or a precursor thereof, forming adefinitively-shaped compact or "green body" from the powder, andsintering the green body to produce a densified hard, tough ceramic. Inthe sintering process, the green body is subjected to high temperatures(about 1800°-2000° C.) which facilitate material transport. Thistransport reduces the pore size and volume between the green bodyparticles and assists in the bonding of adjacent particles, thusproducing a strong, dense ceramic. Ceramics used in high temperatureapplications often require advanced sintering processes, such as gaspressure or pressure-assisted sintering ("GPS") or glass encapsulatedhot isostatic pressing ("hipping" or "HIP" or "HIPping"), which subjectthe green body not only to high temperatures but also to pressures onthe order of 30,000 psi.

It has been found that GPS processes are typically conducive to growinggrains having a high aspect ratio. During GPS, silicon nitride normallytransforms completely from the alpha to the beta phase in theintermediate stages of sintering, during which time beta grain growthand pore elimination occur. Moreover, GPS cycles are typically run attemperatures greater than 1850 degrees C., thus favoring grain growth oflarge beta nuclei. Accordingly, GPS ceramics typically possess hightoughness. However, because a GPS ceramic typically requires at leastabout 10 weight percent ("w/o") of sintering aids, its high temperatureproperties are usually mediocre.

In contrast, when a silicon nitride green body containing less than 5w/o sintering aid is HIPped at about 1825 degrees C., it densifies togreater than 90% of theoretical density in as little as 15 minutes.Although the lower level of sintering aids in such hipped ceramics leadsto superior high temperature properties, little grain growth occurs andmicrostructural development takes place only in the latter stages ofdensification. Since there is little opportunity for the formation oflarge beta nuclei, HIPped silicon nitrides do not possess the highaspect ratio grains required for high toughness.

Previous attempts to grow high aspect ratio grains in hipped siliconnitride included seeding the silicon nitride powder with beta grains andraising the HIPping temperature to at least 2000 degrees C. However,seeding was found to lower the average fracture toughness while the hightemperature hipping modification increased the rate of silicon nitridesublimation, thus yielding ceramics having unacceptable levels ofporosity.

Accordingly, it is the object of the present invention to provide asilicon nitride ceramic having low levels of sintering aids and a hightoughness.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a sinteredsilicon nitride ceramic comprising between about 0.6 mol % and about 3.2mol % rare earth as rare earth oxide, and between about 85 w/o and about95 w/o beta silicon nitride grains, wherein at least about 20 w/o of thebeta silicon nitride grains have a thickness of greater than about 1micron.

DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the effect of indentation load on the strengthof Example D of the present invention.

FIG. 2 demonstrates how the toughnesses of Examples C, D and E of thepresent invention approach 10, 11 and 9 MPa m 1/2, respectively, atcrack lengths of less than 800 microns.

FIG. 3 presents the grain size distribution of Example D of the presentinvention.

FIG. 4 presents the correlation between aspect ratio and thickness forExample D of the present invention.

FIG. 5 presents the grain size distribution for three binary rare earthembodiments of the present invention (Examples L, M & N) along with thatof Comparative Example CA.

FIG. 6 presents the correlation between high aspect ratio grainfrequency ("linear density") and toughness for high toughness binaryrare earth embodiments of the present invention (squares) and a singlerare earth (triangle).

DETAILED DESCRIPTION OF THE INVENTION

It has been unexpectedly found that the toughness of hipped siliconnitrides can be increased by either (a) subjecting the hipped ceramic toa post-hip heat treatment, or (b) presintering the green body at atemperature of at least about 2000 degrees C. Without wishing to be tiedto a theory, it is believed that the presintering thermal treatmentenables the formation of large beta nuclei which then grow duringhipping to become modest aspect ratio coarse grains, often withdiameters of more than one micron. Similarly, the post-HIP heattreatment provides energy for the coarsening of thin, high aspect ratiobeta grains formed during HIPping. It is believed the coarse grainsproduced by these thermal treatments provide enhanced toughness throughcrack bridging.

The silicon nitride starting material of the present invention may beany silicon nitride powder or precursor thereof. In preferredembodiments, alpha silicon nitride powder is used as the startingmaterial and comprises between about 95 w/o and about 99 w/o by weightof the final ceramic body. More preferably, the silicon nitride powderis at least about 80 w/o alpha phase, has a surface area of betweenabout 5 and about 20 m² /g , a silica content of between about 1.5 w/oand about 3.0 w/o, and an iron content of less than about 100 ppm. Mostpreferably, the silicon nitride starting material is a mixture of UbeE03, Ube E05 and Ube E10, each available from Ube Industries, New York,N.Y.

The present invention comprises at least one rare earth sintering aid toassist the densification of the silicon nitride. Typically, the rareearth comprises between about 0.6 mol % and about 3.2 mol %, preferablybetween about 1.2 mol % and about 2.4 mol %, as rare earth oxide of thesintered ceramic. In some embodiments, this rare earth sintering aid isselected from the group consisting of yttria, samaria and ytterbia, andpreferably comprises about 2.4 mol % of the ceramic as rare earth oxide.In more preferred embodiments, the present invention comprises twosintering aids selected from the group consisting of samaria, neodymiaand erbia. In some binary embodiments, each rare earth typicallycomprises at least about 0.3 mol % of the ceramic as rare earth oxideand between about 30 w/o and about 70 w/o of the total rare earthcomponent.

In some embodiments of the present invention, silica can be added to themixes of silicon nitride and rare earth starting materials. Addingbetween about 1 w/o and about 3 w/o silica provides a rareearth-to-silica ratio conducive to formation of the rare earthdisilicate grain boundary phase.

The silicon nitride starting material and sintering aids can beprocessed into green bodies in any conventional manner. Methods used inmixing the powder and aids ("the mixes") of the present inventioninclude, but are not limited to, ball milling and attrition milling. Insome embodiments, the mixes are milled in water. In other embodiments,the mixes are milled in isopropyl alcohol for 36 hours and subsequentlydried. The preferred milling media is either polyurethane ornylon-coated steel balls.

If the mixes have a high surface area, they may be spray dried to createa flowable powder.

The mixes may be filtered for impurities at any stage of the process.Typically, filter is undertaken after the milling step and is usuallycarried out with a 20 micron filter.

Temporary binders used to assist in green body formation include, butare not limited to, polyethylene glycol and PVA.

Green body forming methods include die pressing, rubber mold pressing,extrusion molding, slip casting, and injection molding. In someembodiments, the mixes are cold pressed into tiles and then coldisostatically pressed at 30 ksi.

If an organic binder is used to assist green body formation, the greenbody may be air fired at about 600 degrees C. to remove the binder.

In some embodiments, the green body is presintered at a temperature ofbetween about 1750 degrees C. and about 2000 degrees C. in about0.14-105 MPa nitrogen for about 30 to 180 minutes. This presinteringstep accelerates the formation of large beta nuclei which then growduring hipping to become grains of modest aspect ratio, often withthicknesses of more than one micron. After a green body is presintered,its density is typically between about 1.6 g/cc and about 2.5 g/cc.

If the green body is presintered as above, it is typically subjected toa HIPping step in accordance with U.S. Pat. No. 4,446,100. Typically,the HIPping includes a soak at a temperature of between about 1800 andabout 1900 degrees C. for about 60 minutes. During the HIPping step, thelarge beta nuclei produced by the presinter step grow rapidly at theexpense of the smaller grains surrounding them. After HIPping, thesintered ceramic typically has a density in excess of about 99.5% oftheoretical density.

In other embodiments of the present invention, the green body issubjected to HIPping in accordance with U.S. Pat. No. 4,446,100 and to asubsequent heat treatment in nitrogen. The post hip heat treatmentcoarsens the microstructure of the HIPped ceramic. Typically, thepost-HIP heat treatment is undertaken at a temperature of between about1800 and about 2000 degrees C. in a 50-250 MPa nitrogen atmosphere forbetween about 2 and about 4 hours. More preferably, this treatment canbe undertaken at about 1840 degrees C. in a 200 MPa nitrogen atmospherefor about 4 hours. The post-hipping heat treatment may optionally beundertaken in a powder bed of silicon nitride.

It is also contemplated that the results of the post-HIP heat treatmentcan also be realized by merely extending the length of the soak in theHIP operation. For example, it is contemplated that extending theconventional hipping operation to about 4 hours may achieve similarproperties.

The microstructure of the ceramics of the present invention typicallyreveals beta grains comprising at least about 85-95 w/o of the ceramic.Typically, the mean thickness of the beta grains is between about 0.3and about 1.0 micron, and the mean aspect ratio is between about 6 andabout 10.

In most preferred embodiments, the sintered ceramics described abovetypically have a flexural strength of at least about 350 MPa at 1371degrees C., a compressive creep rate of less than about 10⁻⁶ /sec at1371 degrees C. and 200 MPa, and a toughness which increases to about9-11 MPa m^(1/2) within the first 600 to 800 microns of the crack. Thelatter characteristic is termed R-curve behavior.

EXAMPLES A-G

The first seven examples examined the effect of subjecting the hippedceramic to a post-hip heat treatment on toughness. A combination of UbeE03 and E10 silicon nitride powder was used as the silicon nitridestarting material. The average particle size of the combination was lessthan about 1.5 microns, with 90% being finer than 3.0 microns; itsoxygen content examination was about 1.0 w/o; its iron content was about100 parts per million (ppm); its boron content was about 50 ppm; and itsBET surface area was between about 8 and 9 m² /g.

This silicon nitride starting material was mixed with rare earth oxidesso that the total rare earth oxide content was 0.0177 moles per 100grams of powder (i.e., about 2.4 mol% of the hipped ceramic). The rareearth oxides used in these Examples were as follows:

    ______________________________________                                        Example Rare Earth #1 (w/o)                                                                           Rare Earth #2(w/o)                                    ______________________________________                                        A       erbia     3.44      yttria  1.97                                      B       yttria    2.00      neodymia                                                                              2.98                                      C       samaria   3.08      erbia   3.44                                      D       samaria   3.08      erbia   3.44                                      E       neodymia  2.98      ytterbia                                                                              3.49                                      F       yttria    4.00                                                        G       yttria    4.00                                                        ______________________________________                                    

In Examples, A and C, 2.13 w/o of silica was also added. This mix wasthen ball milled in water. The milled product had a solids content ofbetween about 50-65 % , a surface area of between about 8 and 11 m² /g,and a mean particle size of about 0.9 microns. The milled slip was thenfiltered through a 20 micron filter. A binder comprising about 3%Carbowax 8000, available from Union Carbide of Danbury, Conn. and 0.5%PVA was then blended into the slip and the slip was spray dried. Thespray-dried powder was diepressed at 5000-10000 pounds into a three inchby three inch tile. The pressed tile was then isopressed to 30 ksi. Thebinder was burned out in air at 600 degrees C. The tile was then hippedat 1840 degrees C. for 60 minutes. Post-hip heat treatment was thencarried out as follows:

    ______________________________________                                        Example     post-hip soak temp                                                                          time (hr)                                           ______________________________________                                        A           2000 C.       2                                                   B           2000 C.       2                                                   C           1840 C.       5                                                   D           1840 C.       3                                                   E           1840 C.       5                                                   F           1840 C.       5                                                   G           1840 C.       3.3                                                 ______________________________________                                    

Toughness was determined by the indentation fracture method, asdescribed in Krause, "Rising Fracture Toughness From the BendingStrength of Indented Alumina Beams", J. Am. Cer. Soc. 71(5) 338-43(1988). The indent load range between about 2.5 and about 50 kg. Flexurestrength testing of the ceramics of the present invention was undertakenon 4 point quarter point fixtures with an outer span of 40 mm. Theflexure bar size was either 3 mm by 4 mm by 50 mm or 3 mm by 4 mm by 25mm.

The effect of load on the strength of Example D of the present inventionis presented in FIG. 1. The slope of the strength/load curve for ExampleA-G is presented below:

    ______________________________________                                               Example                                                                              Slope                                                           ______________________________________                                               A      -0.30                                                                  B      -0.31                                                                  C      -0.25                                                                  D      -0.26                                                                  E      -0.27                                                                  F      -0.26                                                                  G      -0.31                                                           ______________________________________                                    

If the slope of the strength versus load curve is less than about -0.33,the ceramic exhibits R-curve behavior,i.e., the toughness of the ceramicincreases with crack length. Because each ceramic post-HIPped at 1840°C. has a slope of less than about -0.33, each has R-curve behavior. Thetoughness at a crack length of about 800 microns for Examples C, E-G wasat least about 9 MPa m^(1/2), while the toughness at a crack length ofabout 800 microns for Example D was 11 MPa m^(1/2). The R-curves forExamples C, D and E are shown in FIG. 2.

Microstructural analysis of Example D was undertaken. This was done bypolishing and then etching the samples in molten KOH. Micrographs weretaken at 2500X and 5000X. The 5000X photos were used to estimate thegrain thickness. Four to six lines were drawn across the micrographs.The shortest dimension of any hexagonal or rectangular shaped grainsintercepted by the lines were measured. See FIG. 3, in which the graindistribution of Example D is presented. It is observed that at leastabout 20% of the grains of Example D have a thickness of at least about1 micron. The correlation between aspect ratio and grain thickness wasexamined in Example D. Aspect ratios were determined by drawing fourlines across a 2500x micrograph of Example D and recording the thicknessand length of each rectangular grain which touched a line. The resultingdependence of aspect ratio on grain thickness for Example D is presentedin FIG. 4. FIG. 4 reveals that about 50% of these grains have athickness of more than 1 micron and those grains have rather modestaspect ratios (i.e., between about 4 and about 7). At least about 45% ofthe beta silicon nitride grains have an aspect ratio of at least 6. Inconventional HIPped silicon nitride ceramics, over 90% of the grainshave a thickness of less than 1 micron.

Examples H and I

The following two Examples examined the effect of presintering ontoughness. The rare earth content of Examples H and I were as follows:

    ______________________________________                                        Example Rare Earth #1 (w/o)                                                                           Rare Earth #2(w/o)                                    ______________________________________                                        H       samaria   3.09      ytterbia                                                                              3.49                                      I       samaria   3.08      erbia   3.44                                      ______________________________________                                    

The processing of Examples H and I were carried out exactly as describedabove except that each green body was subjected to a presinter at 2000degrees C. at 1500 psi nitrogen for 30 minutes and there was no post-hiptreatment.

Strength and toughness measurements were carried out exactly as inExamples A-G above. The slope of the strength/load curve for Examples Hand I were -0.27 and -0.28, respectively, thus indicating R-curvebehavior. The toughness at a crack length of about 800 microns forExamples H and I was about 8 MPa m^(1/2).

Alternative methods of increasing the toughness of HIPped siliconnitrides have also been discovered. It has been observed that binaryrare earth sintering systems yield a finer grain diameter and a higheraspect ratio microstructure than single rare earth systems. It isbelieved these qualities increase the toughness of the ceramics throughcrack deflection mechanisms, whether or not the ceramic is subjected topresinter or post-HIP thermal treatments. Examples of this phenomenonare set out below:

EXAMPLES J-T

Examples J-T were processed identically to Examples A-I, except thatthere was neither a presinter nor post-hip heat treatment step, and therare earth contents were as follows:

    ______________________________________                                        Example Rare Earth #1 (w/O)                                                                           Rare Earth #2 (w/o)                                   ______________________________________                                        J       erbia     3.44      yttria  1.97                                      K       erbia     4.00      yttria  1.64                                      L       lanthia   2.31      ytterbia                                                                              4.19                                      M       lanthia   2.02      yttria  2.60                                      N       neodymia  2.98      ytterbia                                                                              3.49                                      0       yttria    2.00      neodymia                                                                              2.98                                      P       samaria   3.09      ytterbia                                                                              3.49                                      Q       samaria   3.09      ytterbia                                                                              3.49                                      R       samaria   3.08      erbia   3.44                                      S       samaria   3.08      erbia   3.44                                      T       neodymia  2.98      ytterbia                                                                              3.49                                      ______________________________________                                    

Each of the above Examples was selected so that the grain boundary phasewould contain two rare earth disilicate phases at equilibrium. Inaddition, two single rare earth systems, Examples CA and CB, wereprocessed identically to Examples J-T, in order to study the effect ofthe binary systems on grain size and toughness. Their compositions wereas follows:

    ______________________________________                                        Example Rare Earth #1 (w/o)                                                                           Rare Earth #2(w/o)                                    ______________________________________                                        CA      yttria    4.00      none                                              CB      samaria   6.17      none                                              ______________________________________                                    

Toughness of the Examples J-T was also examined as per the method chosenfor Examples A-I. The toughnesses, which were found to be independent ofcrack length, are presented below:

    ______________________________________                                        Example     Toughness (MPa m1/2)                                              ______________________________________                                        J           6.01                                                              K           5.71                                                              L           6.23                                                              M           5.78                                                              N           5.87                                                              O           6.25                                                              P           6.26                                                              Q           6.57                                                              R           not tested                                                        S           not tested                                                        T           not tested                                                        CA          5.46                                                              CB          3.71                                                              ______________________________________                                    

The selected binary rare earth sintering systems provide highertoughness than single rare earth sintering aid systems. One particularcombination of the binary rare earth sintering aid systems provides ahigher level of toughness than the other binary rare earth systems. Inparticular, the samaria/ytterbia system (Example Q) produces a toughnessof at least about 6.5 MPa m^(1/2) in sintered silicon nitride ceramicswhile the other binary systems examined provide no more than about 6.26MPa m^(1/2). Although, this phenomenon has only been examined inembodiments having a standard hipping cycle (i.e., no presinter norpost-HIP heat treatment), it is believed that the samaria/ytterbiasystem would produce a similarly superior toughness in embodimentscontaining either the presinter or post-hip heat treatments.

Microstructural analysis of Example Nos. J-T was undertaken as above.FIG. 5 presents the distribution of grain thicknesses for three of thebinary rare earth systems (Examples L, M and N) along with a ComparativeExample CA. Whereas the ceramics having a binary system have a mediangrain thickness of about 0.3 microns, the single rare earth system has amedian grain thickness of about 0.6 microns.

The aspect ratio of the grains was estimated from 2500X micrographs.Four lines were drawn across the micrographs. The thickness and lengthwere recorded for each rectangular grain touching a line. This result isan estimate of the aspect ratio distribution. The frequency ("lineardensity") of high aspect ratio grains (i.e., grains having an aspectratio greater than 4) was found to correlate with toughness. Thiscorrelation is presented in FIG. 6.

The compressive creep of Example No. Q was examined on a cylinder (0.263inch diameter, 0.5 inch length) loaded between two silicon carbide rodsat 1371 degrees C. and 200 MPa. Four silicon nitride flags werecantilevered on the cylinders. The distance between the flags wasmeasured by a non-contact laser extensometer system. The compressivecreep of Example No. Q was observed to be 10⁻⁶ /sec. Because this levelof creep resistance is similar to that found in conventional siliconnitride ceramics using only a single rare earth sintering aid (e.g.,NT-154, manufactured by the Norton Company, Worcester, Mass.), utilizinga binary rare earth system in a hipped silicon nitride ceramic likelyincreases its toughness without degrading its high temperatureproperties.

It has also been observed that certain binary rare earth systems formtwo distinct phases within the grain boundary region of a sinteredsilicon nitride ceramic, i.e., a first phase consisting essentially of afirst rare earth disilicate and the second phase consisting essentiallyof a second rare earth disilicate. Conventional rare earth systems tendtowards a single disilicate phase in the grain boundaries of a sinteredsilicon nitride ceramic. For example, it is well known that a singlerare earth oxide tends towards a single disilicate phase in the grainboundary regions. Similarly, a pair of rare earths having similar atomicradii likely form a solid solution at equilibrium and so tends towards asingle disilicate phase also. Lastly, a pair of rare earths havingdissimilar atomic radii but utilized in amounts which ,at equilibrium,fall outside the two phase region also tends towards a single disilicatephase in the grain boundary regions. Because there are no other phasesin the grain boundary to compete with, these predominant disilicatephases tend to form large (i.e., about 1 mm) disilicate crystals. Whenthese crystals become this large, they can act as severe toughness andstrength limiting flaws. In contrast, when rare earths are selected fromdifferent lanthanide groups (and so have dissimilar atomic radii) andare utilized in amounts which fall within the two phase disilicateregion at equilibrium, two competing disilicate phases are produced withneither disilicate phase being able to form large crystals which limitstrength, i.e, the presence of a second disilicate phase prevents eitherdisilicate phase from growing too large and acting like a strengthlimiting flaw. In these embodiments, the rare earth oxides havingdissimilar radii are selected from two of the three lanthanidesubgroups, such subgroups comprising:

a) lanthanum, cerium, praesodynmium, neodymium, promethium, samarium andeuropium;

b) gadolinium, terbium, dysprosium and holmium; and

c) yttrium, erbium, thulium, ytterbium, and lutetium,

wherein the amounts of each rare earth oxide are selected so that thesilicon nitride/binary rare earth mixture forms two rare earthdisilicate phases at equilibrium. When a binary rare earth sintering aidsystem is used in accordance with the present invention, thesecrystalline phases are not visible at 200X. Although this dual phasephenomenon has only been examined in embodiments having a standardhipping cycle (i.e., no presinter nor post-HIP heat treatment), it isbelieved that the dual phase could also exist in embodiments containingeither the presinter or post-HIP heat treatments.

It has also been observed that when the binary rare earth system isselected from different lanthanide groups (and so have dissimilar atomicradii) and are utilized in amounts which fall within the two phasedisilicate region at equilibrium, rapid cooling of the hipped ceramiccan lead to an amorphous grain boundary phase. Without wishing to betied to a theory, it is believed that the formation of two rare earthdisilicate phases in the grain boundary phase requires much moreordering and therefore much more energy and time than a single rareearth disilicate phase. When insufficient cooling time is given to suchbinary rare earth systems, an amorphous phase is formed. Because theamorphous grain boundary phase provides low residual stress, theceramics produced in these embodiments are thought to be useful in highstress applications such as bearing applications which require a veryuniform microstructure.

EXAMPLES U-X

The ability of binary rare earth systems to form both amorphous andcrystallite phases is presented in Examples U-X. Ube E03, E05 and E10silicon nitride powder was used as the silicon nitride startingmaterial. This silicon nitride starting material was mixed with rareearth oxides so that the total rare earth oxide content was 0.0177 molesper 100 grams of powder. The rare earths used in these Examples were asfollows:

    ______________________________________                                        Example Rare Earth #1 (w/o)                                                                           Rare Earth #2(w/o)                                    ______________________________________                                        U       ytterbia  4.09      lanthia                                                                              2.25                                       V       ytterbia  4.85      lanthia                                                                              1.61                                       W       yttria    2.58      lanthia                                                                              2.01                                       X       yttria    1.60      lanthia                                                                              3.40                                       ______________________________________                                    

These Examples were selected to provide dual rare earth disilicatephases at equilibrium. In addition, two single rare earth systems,Comparative Examples CC and CD, were processed identically to ExamplesU-X, in order to study the effect of the binary systems on crystallitesize. Their compositions were as follows:

    ______________________________________                                        Example        Rare Earth #1 (w/o)                                            ______________________________________                                        CC             ytterbia  6.78                                                 CD             lanthia   5.67                                                 ______________________________________                                    

Each of these mixtures was ball milled for 36 hours in isopropanol at aconcentration of 45-50 w/o solids to form a milled slurry having a finalsurface area of about 8-11 m² /g and a mean particle size of about 0.6microns. This milled slurry was then wet screened through a 400 meshscreen. Next, the isopropanol was evaporated in a friction air dryer,yielding a dry powder. The powder was then pressed into a three inchsquare tile to facilitate handling. The tiles were then sequentiallysubjected to cold isostatic pressing at 30,000 psi; presintered at 1450degrees C.; hipped at a temperature about 1840 degrees C. at 25 to 30ksi for 60 minutes to form ceramic bodies; and then rapidly cooled at arate of between about 700 and about 1000 degrees C. per hour until theceramic reached a temperature of about 600 degrees C.

It was observed that none of the hipped Examples U-X possessed acrystallized grain boundary phase (i.e., each had an amorphous grainboundary phase). In contrast, XRD analysis of Comparative Examples CCand CD revealed that each possessed a crystallized grain boundary withcrystals on the order of 1 mm in diameter.

After determining that each of Examples U-X possessed an amorphous grainboundary phase, each was subjected to a post-hip heat treatment innitrogen at about 1400 degrees C. for about 10 hours. XRD analysis ofthese heat-treated Examples revealed that the grain boundaries hadcrystallized into unquantifiably small sized crystals. Furthermeaningful analysis of the size and type of the grain boundary phases ofExamples U-X could not be undertaken. Because crystal size wasunquantifiable via XRD analysis of Examples U-X, it can be reasonablyconcluded that these Examples contained much smaller crystals and thatthe small size can be attributed to the competition between the dualdisilicate phases.

When HIPping is used for densification, the ceramics of the presentinvention may be made into shapes sufficiently complicated such that theshape can not be produced by hot pressing.

I claim:
 1. A sintered silicon nitride ceramic comprising:a) acrystalline phase comprising between about 85 w/o and about 95 w/o betasilicon nitride grains, and b) a grain boundary phase consistingessentially ofi) between about 0.6 mol % and about 3.2 mol % rare earth,as rare earth oxide, and ii) no more than 2.0 w/o excess oxygen, assilica, wherein at least about 20% of the beta silicon nitride grainshave a thickness of greater than 1 micron.
 2. The ceramic of claim 1having a density of at least about 99.5% of theoretical density.
 3. Theceramic of claim 2 having an initial toughness of at least 8 MPam^(1/2).
 4. The ceramic of claim 3 wherein the toughness increases to atleast 9.0 MPa m^(1/2) at a crack length of 600 microns.
 5. The ceramicof claim 4 wherein the rare earth component comprises neodymium andytterbium.
 6. The ceramic of claim 4 wherein the rare earth componentcomprises about 3 w/o neodymium and about 3.5 w/o ytterbium, as rareearth oxides.
 7. The ceramic of claim 1, wherein at least about 50% ofthe beta silicon nitride grains have a thickness of greater than 1micron.
 8. The ceramic of claim 7 wherein at least about 45% of the betasilicon nitride grains have an aspect ratio of at least
 6. 9. Theceramic of claim 8 having an initial toughness of at least 9 MPam^(1/2).
 10. The ceramic of claim 9 wherein the toughness increases toat least 11.0 MPa m^(1/2) at a crack length of 600 microns.
 11. Theceramic of claim 10 wherein the rare earth component comprises samariaand erbia.
 12. A sintered silicon nitride ceramic comprising:a) acrystalline phase comprising between about 85 w/o and about 95 w/o betasilicon nitride grains, and b) a grain boundary phase consistingessentially ofi) between about 0.6 mol % and about 3.2 mol % rare earth,as rare earth oxide, and ii) between about 1.5 and about 4.0 w/o excessoxygen, as silica, wherein the ceramic has an initial fracture toughnessof at least about 8 MPa m^(1/2).
 13. The ceramic of claim 12 wherein atleast about 20% of the beta silicon nitride grains have a thickness ofgreater than 1 micron.
 14. The ceramic of claim 13 wherein at leastabout 45% of the beta silicon nitride grains have an aspect ratio of atleast
 6. 15. The ceramic of claim 14 wherein at least about 50% of thebeta silicon nitride grains have a thickness of greater than 1 micron.16. The ceramic of claim 15 having an initial fracture toughness of atleast 9 MPa m^(1/2).
 17. The ceramic of claim 16 wherein the toughnessincreases to at least 11.0 MPa m^(1/2) at a crack length of 600 microns.18. The ceramic of claim 17 wherein the rare earth component comprisessamaria and erbia.